The research outlined in this proposal is designed to examine the manner and extent to which factors such as an organism's prior experience, the type of reinforcer and the current environmental setting determine the behavioral effects of abused drugs. A variety of different behaviors will be experimentally established and maintained by the scheduled presentation of termination of events such as food, shock and drug injection, as well as by the presentation of stimuli associated with the occurrence of these events (second-order schedules). Specific experiments will focus on: 1) identifying relevant features in the organism previous experience which significantly modify the typical effects a drug will have a behavior; 2) examining the influence of the more immediate environmental context on the behavioral effects of drugs; and 3) assessing the extent to which the type of reinforcing event maintaining responding determines drug effects. This research will provide essential information directly relevant to an adequate understanding of how identifiable historical and prevailing environmental variables directly influence the behavioral effects of abused drugs.